The present application relates to battery powered electrical devices for use in hazardous locations. While it finds particular application to hand-held and other readily transportable devices, the application also relates to stationary, battery-backed, and other battery powered electrical devices suitable for use in environments which present a risk of fire or explosion.
Battery powered electrical devices are ubiquitous. Indeed, such devices are widely used in home, commercial, industrial, and other environments to perform a wide variety of functions. Unless specifically designed, however, such devices are not typically suited for use in hazardous locations.
Hazardous (classified) locations include those locations in which ignitable concentrations of flammable or combustible materials are or may reasonably be expected to be present in the atmosphere. Such locations can be encountered, for example, in petrochemical, mining, agricultural, and industrial facilities. Depending on the classification scheme, hazardous locations may be classified in various ways. In North America, for example, a Class I, Division 1 hazardous location is a location where ignitable concentrations of flammable gases, vapors or liquids can exist under normal operating conditions, may frequently exist because of repair or maintenance operations or because of leakage, or may exist because of an equipment breakdown that simultaneously causes the equipment to become a source of ignition. Under a classification scheme which is used outside of North America, a Zone 0 hazardous location is a location where an explosive gas-air mixture is continuously present or present for long periods.
Various techniques have been used to render electrical equipment suitable for use in hazardous locations. One technique involves the use of explosion-proof housings. An explosion proof housing is designed to withstand an explosion occurring within it and to prevent the ignition of combustible materials surrounding the housing. Explosion-proof housings also operate at an external temperature below that which is sufficient to ignite surrounding materials. While explosion-proof housings can be quite effective, they tend to be both expensive and physically large, rendering them relatively unattractive for use in applications in which cost or physical size is a factor.
Another technique involves the use of purging, in which an enclosure is supplied with a protective gas at a sufficient flow and positive pressure to reduce the concentration of a flammable material to an acceptable level. However, purging systems can be relatively complex, and a source of purge gas may not readily available.
Another technique involves the use of intrinsically safe electrical circuits. Intrinsically safe circuits are typically energy limited so that the circuit cannot provide sufficient energy to trigger a fire or explosion under normal operating or fault conditions. One definition of an intrinsically safe circuit which is sometimes used in connection with the certification of intrinsically safe equipment is contained in Underwriters Laboratory (UL) Standard 913, entitled Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations. According to this definition, an intrinsically safe circuit is one in which any spark or thermal effect, produced normally or in specified fault conditions, is incapable, under the test conditions proscribed in [the UL 913] standard, of causing ignition of a mixture of a flammable or combustible material in air in the mixture's most easily ignitable concentration.
Various intrinsically safe battery powered electrical devices have been produced. Examples include flashlights, laser pointers, scales, digital voltmeters (DVMs), radios, clocks, and wall thickness monitors. One flashlight has included three (3) light emitting diodes (LEDs) each having a nominal forward voltage of about 3.6 volts direct current (VDC). The flashlight has been powered by three (3) 1.5 VDC Type N batteries, with an energy limiting resistor disposed electrically in series between the batteries and the LEDs. A particular disadvantage of such a configuration, however, is that three (3) batteries are required to supply the nominal 3.6 VDC forward voltage of the LEDs. A still further disadvantage is that the current supplied to the LEDs is a function of the battery voltage, the LED forward voltage, and the series resistance. As a result, the intensity of the light produced by the flashlight can vary significantly as the batteries discharge. Moreover, such a configuration utilizes the energy from the batteries relatively inefficiently, so that the flashlight is relatively bulky for a given light output and operating time.
Other intrinsically safe flashlights have included an incandescent or halogen bulb powered by two (2) AAA batteries, again connected electrically in series through a current limiting resistor. This configuration again suffers from variations in light intensity and a relatively inefficient utilization of the available battery energy. While the bulbs can be operated on the voltage supplied by only two batteries, devices containing such bulbs cannot readily be certified for use in Class I, Division I locations, thereby limiting their utility.
Still other devices have been powered by intrinsically safe lithium ion (Li Ion) batteries having a nominal voltage of about 3.6 VDC. While the relatively higher output voltage of these batteries provides additional application flexibility, they tend to be less widely available than other battery types.
Moreover, some devices require multiple voltage and/or current supplies. While it is possible to provide different batteries for powering different parts of the device or to connect multiple batteries in series to provide different output voltages, it is generally desirable to reduce the number and/or types of batteries required to power a particular device.